Orthogonal frequency division multiplexing is widely used technique of transmission in the RF domain where it allows mitigating of signal fading in multi-path propagation. The present invention discloses the use of orthogonal frequency division multiplexing in optical links and, in particular, in fiber communications using dual-polarization transmission.
In optical OFDM each channel the optical carrier is directly modulated by a complex RF signal that can be construed as a linear combination of M separate digitally modulated RF signals at frequencies fm such that fm=m/T where T is the period of modulation. Thus the total symbol rate of the transmitted information is M/T. In the text we shall refer to the frequencies fm as “sub-carriers”.
In modern communication systems, a coherent detection technique is implemented, which provides improved sensitivity compared with traditional direct detection schematics. Typically coherent detection is used for phase-shift-keying (PSK) data transmission. The present invention is also focused on PSK, and in the preferred embodiment, QPSK (quadrature PSK) data transmission. However this does not limit the scope of the invention, and various types of data modulation can benefit from the disclosed invention.
In coherent receiver, the QPSK incoming optical signal is mixed with a strong local oscillator signal to produce in-phase (I) and in-quadrature (Q) outputs. I and Q components of the output optical signal are converted into electrical signals by a set of photodetectors. In the preferred configuration four balanced photodetectors are used to recover QPSK encoded data.
Data transmission using light of two orthogonal polarizations via the same optical channel allows doubling the data rate. At the receiver side, the optical signal is split by a polarization beam splitter, and the light of each orthogonal polarization is mixed with a local oscillator signal of the corresponding polarization in the coherent receiver.
However the orthogonality of the optical signal polarization is not preserved when the signals are transmitted via fiber link. The received optical signals neither orthogonal to each other, nor aligned with the polarization beam splitter at the receiver side. The present invention addresses this problem of the polarization state recovery in dual-polarization data transmission.
U.S. Patent Application Pub. No. 20070092259 discusses implementation of polarization compensation in the coherent optical receiver. The successful recovery of the I and Q components of the received signal is performed based on filtering using an inverse Jones matrix in combination with continuous high-speed adaptive training of the polarization compensator. In order to ease the processing, the signal is split into a number of parallel sub-streams with reduced data rate.
There is a need to develop a polarization compensation technique and method for optical orthogonal frequency division multiplexed communications.